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This study was designed to evaluate the way in which changes in vertical jaw opening affected the relative contributions of various masticatory muscles to bite force production. EMG activity was recorded simultaneously from the masseter, and anterior, middle, and posterior temporalis muscles, during controlled isometric biting at different force levels and vertical jaw openings. EMG-force characteristics were compared between muscles and bite openings. All but the posterior temporalis muscle displayed significant increases in muscle activity with increased bite force production; the masseter muscle demonstrated the largest activity increments. Statistically significant changes in muscle function due to jaw opening were demonstrated only for the masseter muscle, though similar trends were observed for the anterior and middle temporalis muscles. Minimum increases in muscle activity associated with increases in bite force occurred between 9 and 11 mm of opening, measured at the first molar, for all three muscle groups. The results of this study suggest that changes in masticatory muscle length resulting from vertical jaw opening cause alterations in muscle contractile properties, but the relative contributions of various masticatory muscles toward bite force production may also be affected by biomechanical factors and neural control adaptations.

The objective of this experiment was to determine the relative contributions and patterns of activity of different muscles involved during the oral phase of swallowing. Electromyographic (EMG) signals were recorded from the orbicularis oris inferior, masseter, palatal elevator, anterior and posterior genioglossus, mylohyoid, anterior belly of the digastric, and vocalis muscles of 12 normal adult subjects. Each subject swallowed 15 mL of water, under normal and bite block conditions, 15 to 20 times. The integrated EMG signals for each subject's swallows were ensemble averaged. The results of the analyses showed that swallowing function varies from individual to individual in terms of the specific muscles used and how the various muscle activity patterns are coordinated. These results suggest that swallowing is a highly complex adaptive motor activity which probably relies more on higher-level control mechanisms than previously believed.

This study was undertaken to determine the activation and coordination patterns of the three suprahyoid muscles--geniohyoid, mylohyoid, and anterior belly of the digastric muscle--in elevating the larynx during swallowing. Electromyographic activity was also recorded from two intrinsic laryngeal muscles (vocalis and lateral cricoarytenoid) and the anterior genioglossus. Ten adults served as participants. Each participant produced 15 swallows of 15 mL of tap water both normally and with a 12-mm bite block placed between the molars. The electromyographic data were ensemble-averaged with a laboratory computer. Analyses showed that the three suprahyoid muscles were used selectively by different participants. Some participants used all three muscles for hyoid elevation, while others used different pairs of two of the muscles. The activation patterns of the suprahyoid muscles during swallowing also varied with respect to each other and the onset of the laryngeal constrictor muscles; however, use of at least one suprahyoid muscle always preceded the onset of the laryngeal adductors, indicating that larynx elevation consistently preceded glottal adduction. The way in which the muscles responded to the bite block varied considerably both within and among participants. Some maintained temporal stability but increased overall muscle activity; others reorganized temporal relations either with or without corresponding muscle activity adjustments. These findings suggest that the laryngeal elevation system is an adaptive function rather than an immutable action.

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In this study, dynamic imaging was used to track the movements of oral cavity and laryngeal structures during swallowing in 10 normal adults subjects. The movements of tiny lead pellet markers attached to the lips, tongue, mandible, and soft palate, as well as anatomic landmarks on the hyoid bone, were measured in relation to a reference pellet affixed to the upper central incisors. Sagittal views of the oral cavity were obtained using standard videofluorography. Each subject produced 10 swallows of 12 ml of tap water followed by 5 swallows with a bite block placed between the molars. The recorded video images were input to a microcomputer where the x- and y-coordinates of the pellets were measured. Results of the analyses revealed considerable temporal overlap in the timing of oral cavity and laryngeal movements, widespread individual variability in coordination patterns and movement trajectories, and selective effects of the bite block. These data suggest the existence of individual adaptive strategies in the programming and control of swallowing movements.

The purpose of this investigation was to examine the utility of EMG-force characteristics as reliable and valid parameters by which to assess oral function. Masseter-muscle activity was recorded during controlled isometric biting exercises performed at various bite openings and force levels on two separate occasions. Measured muscle-activity levels, as well as the EMG-force characteristics of slope and y-intercept, were compared between bite openings and experimental trials. Sensitivity, assessed as the ability to detect statistically significant differences in muscle function due to changes in muscle length, was found to be acceptable for all measurements, except the y-intercept parameter, and during production of minimal forces. Reproducibility, evaluated as the correspondence between values recorded at different experimental sessions, was best for the EMG-force parameter of slope and during production of higher forces. The results from this study support the following conclusions: (1) Substantial variations in muscle function exist between individuals; (2) acceptable reliability and sensitivity of quantitative EMG values can be achieved, especially at higher muscle-activity levels, by rigidly controlling and quantifying functional activities during experimental trials; (3) the slope of an EMG-force curve is a reproducible, quantitative, and functionally sensitive measurement for assessment of muscle function; and (4) the EMG-force y-intercept is not a reliable quantity by which muscle-function characteristics may be studied.